The applications of supercritical fluids as solvents for extracting or separating chemicals, foods and beverage products is increasing as seen in both the technical and patent literature. See, e.g., Pat Tap-10 Patent Technology Review: Supercritical Fluids--"Applications and Materials", Business Communications Co. Inc. 25 VanZant St. Norwalk, Conn. 06855. Applications such as the decaffeination of coffee, extraction of herbs, spices, and separating alcohol and water have been described. Among the many advantages of supercritical separation are that the extract is essentially solvent-free when the solution is de-pressurized and the fluid can be re-cycled. Thus, supercritical systems may be operated without substantial waste streams of contaminated solvent.
Despite interest in generally applying supercritical technology within the chemical industry, such processes have usually been applied to extracting non-polar components from naturally occurring substances such as petroleum, seed oils, or the like. It has been found useful to use a co-solvent such as methanol or water to increase the solubility of compounds in supercritical carbon dioxide. See R. T. Marentis, "Steps to Developing a Commercial Supercritical Carbon Dioxide Processing Plant" ACS Symposium Series, #366 127-143, (1988).
In practical terms, a major obstacle to the commercialization of carbon dioxide-based supercritical processes for selected systems, especially functional aromatic systems which contain compounds suitable for use as monomer reactants, is that functional aromatic compounds of interest tend to have relatively low solubility at moderate pressures.
Although extractions may occur at higher pressures, the increased cost of capital for high pressure hardware and higher energy costs incurred in achieving the high pressures and recycling the gas to these pressures, has made high pressure extraction prohibitively expensive.
Krukonis et al, J. Chem.. Eng. Data, 1985, 30, 247-249, report a difference of roughly two orders of magnitude in the solubility of isomers of hydroxybenzoic acid in carbon dioxide at approximately 200+ atmospheres and 100.degree. C.; the ortho and meta hydroxybenzoic acids being much more soluble than the para isomer. Others have reported rapid separations of isomers of hydroxy-benzoic acid using supercritical carbon dioxide chromatography with modified fluids. See Berger and Deye "Separation of Hydroxybenzoic Acids by Packed Column Supercritical Fluid Chromatography using Modified Fluids with Very Polar Additives", J. Chromatogr. Sci, 29, 26-30 (1991) and "Separation of Benzene Polycarboxylic Acids by Packed Column Supercritical Fluid Chromatography Using Methanol-Carbon Dioxide Mixtures with Very Polar Additives", J. Chromatogr. Sci 29, 141-146 (1991). The foregoing articles report the use of carbon dioxide/methanol mixtures as the supercritical fluid in chromatographic systems with highly polar additives such as trifluoroacetic acid present.